*Microgrid DOI: http://dx.doi.org/10.5772/intechopen.88812*

*Research Trends and Challenges in Smart Grids*

energy source in the world is solar energy.

**Figure 6.** *Solar power plants.*

**5. AC/DC and hydro microgrid**

Photovoltaic generation is systems, which convert the sunlight directly to electricity as shown in **Figure 6**. PV cell technology is very well established and used extensively in microgrid. The DC output of the connected PV cells is given as input to the inverter and AC power supply given to the load through inverter. The most common renewable

All microgrids are more effective in extracting local energy source is only from solar. The main advantages of these solar power plants are very less time required to design and install the power plant. These plants are highly modular, and it is good alternative for peak load demand. Since solar structures are static and no moving parts which gives no noise power plant status to solar power plants. Solar power plants are portable and mobile because of light weight. Since there is no moving part, PV generation systems are having longer life time which makes one of the

Microturbines and microhydro turbines are gas and hydro electric generators ranging in size from 25 to 500 KW connected with microgrid. These microturbines are very much useful in dealing with peak load demands for the microgrids. Biomass is also another renewable source contributing energy supply to microgrids located in remote villages or urban waste sources. The source materials are scrap lumper, forest debris, certain crops or manure, etc. Geo thermal power plants are recent renewable energy sources generating power from geo thermal energy. Geo thermal energy is a thermal energy stored in the earth is utilized. In future, microgrids are expected to add mini geo thermal power plants in its pool of distributed energy resources. In future, the other renewable energy power supply from tides of the sea and hot hydrogen fusion will be a part of microgrid energy sources.

Microgrids are the most prominent grids to extract various types of distributed renewable energy sources with the distribution grid. The distribution utility side grids are AC grids, since today electrical loads are connected with power electronic devices, and since most of the renewable energy sources are DC power supply, now the DC microgrids concepts are gaining more attraction. The most common

attracting renewable energy sources connected with microgrid.

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grids are AC microgrids which uses standard protection technique's to manage disturbances. But the distributed generation sources for microgrid comprising of solar cell arrays and fuel cells are DC power which needs to convert to AC power through power electronic device. This DC power source in the microgrid is connected to the power supply through inverter to the utility grid. The AC power on the utility side is converted back to DC for some of the latest electrical loads such as UPS batteries, DC lighting loads, DC motor drives, and hybrid electric vehicles. Due to multiple conversions, AC microgrid becomes inefficient due to more power losses due to multiple conversions. Apart from that maintain synchronism, stability issues and reactive power requirements are the challenges faced by AC microgrids. Due to this reason, DC microgrids are emerging as alternate grids. But the diesel generator, small microturbines connected synchronous generator, and wind energy generators are required to connect with AC/DC converter to distribution side grid. Due to multiple conversions, DC microgrid also becomes inefficient due to more power losses due to multiple conversions. The reason for this problem is now-a-days electrical load becomes mix of both AC and DC power. It leads AC/DC grids to less efficient operation. The best solution to avoid this multiple conversion energy losses is hybrid AC/DC microgrid. The hybrid AC/DC microgrid is formed with an objective to minimize the conversion losses to make the microgrid more efficient as shown in **Figure 7**. But the implementation of hybrid of AC/DC grid required several technical challenges.

Hybrid microgrid comprises both AC and DC sources as shown in **Figure 7**. The respective AC and DC sources are connected to the corresponding AC and DC networks. The AC bus and DC bus are linked together with three phase converters and the transformers. The AC bus of hybrid grid is tied with distribution grid with transformer and circuit breaker. The successful operations of hybrid microgrid are based on power conditioning converters. The converters are classified based on their input and output power supply. The converters used in hybrid microgrids are rectifier, inverter, buck/boost converter, and transformers. Among these converters, inverters and boost converters need more attention than conventional converters in the microgrid. Because the boost converter is very much required to interface low voltage PV arrays into the microgrid and then converted into AC power through inverter to feed the loads. **Figure 8** shows how the converters are theoretically

**Figure 7.** *Hybrid microgrid.*

**Figure 8.** *Implementation of converters in a typical microgrid.*

implemented; in this Figure, LC stands for load controller and MC for microsource controller. Arrow indicates the electrical load.

### **5.1 Inverter**

Inverter is a power electronic device, which converts DC to AC. The design of the inverter fixes the values of the input voltage, output voltage, frequency, and power output. An inverter is classified into single phase and three phase inverter. The single phase full bridge inverter consists of four switches, and it creates an AC output voltage by switching on and off as shown in **Figure 9**.

Three single three-phase inverters form the three phase inverter. Three-phase inverter has six switches each switch operated at 60 degree point of the fundamental output waveform. Based on the switching frequency and switching pattern, the output voltage, frequency, and power output have been controlled. **Figure 10** shows the general standard equivalent circuit of a three-phase inverter interfaced with solar panels output to microgrid.

#### **5.2 Boost converter**

A boost converter is used as DC to DC power converter to increase the voltage and in turn it will step down the current. In microgrid, one of the major sources is solar energy with DC power supply. The PV panel output voltage is not suitable to feed the loads straightaway. The boost converter is used to step-up the voltage to a required level. **Figure 11** shows the equivalent circuit of boost converter in two modes. One mode switch is closed and other mode switch is open.

Based on the operation of switches, the boost converter operations are divided into two modes. When the switch is closed, the energy is stored in inductor as magnetic energy. Since the diode is switched off, the capacitance is blocked from the power supply. When the switch is open, the inductance is then series with the source which enhances the voltage as boost voltage. The boost voltage is depending on the value of inductor and capacitor.

#### **5.3 Operating modes of the microgrid**

Microgrids are operated in two modes, one is grid connected and another one is islanded mode. The first one is the classical scheme, which is the most

**111**

**Figure 11.**

**Figure 10.**

*Equivalent circuit of a three-phase inverter.*

*Equivalent circuit of a boost converter.*

common mode in use. Microgrids are designed to operate in both modes. Microgrids are designed such a way to get maximum power from renewable energy sources with wind, solar, and microturbines and able to fill its remaining

*Microgrid*

**Figure 9.**

*DOI: http://dx.doi.org/10.5772/intechopen.88812*

*Equivalent circuit of a single-phase full bridge inverter.*

#### **Figure 9.**

*Research Trends and Challenges in Smart Grids*

controller. Arrow indicates the electrical load.

*Implementation of converters in a typical microgrid.*

solar panels output to microgrid.

on the value of inductor and capacitor.

**5.3 Operating modes of the microgrid**

**5.2 Boost converter**

output voltage by switching on and off as shown in **Figure 9**.

modes. One mode switch is closed and other mode switch is open.

**5.1 Inverter**

**Figure 8.**

implemented; in this Figure, LC stands for load controller and MC for microsource

Inverter is a power electronic device, which converts DC to AC. The design of the inverter fixes the values of the input voltage, output voltage, frequency, and power output. An inverter is classified into single phase and three phase inverter. The single phase full bridge inverter consists of four switches, and it creates an AC

Three single three-phase inverters form the three phase inverter. Three-phase inverter has six switches each switch operated at 60 degree point of the fundamental output waveform. Based on the switching frequency and switching pattern, the output voltage, frequency, and power output have been controlled. **Figure 10** shows the general standard equivalent circuit of a three-phase inverter interfaced with

A boost converter is used as DC to DC power converter to increase the voltage and in turn it will step down the current. In microgrid, one of the major sources is solar energy with DC power supply. The PV panel output voltage is not suitable to feed the loads straightaway. The boost converter is used to step-up the voltage to a required level. **Figure 11** shows the equivalent circuit of boost converter in two

Based on the operation of switches, the boost converter operations are divided

Microgrids are operated in two modes, one is grid connected and another one is islanded mode. The first one is the classical scheme, which is the most

into two modes. When the switch is closed, the energy is stored in inductor as magnetic energy. Since the diode is switched off, the capacitance is blocked from the power supply. When the switch is open, the inductance is then series with the source which enhances the voltage as boost voltage. The boost voltage is depending

**110**

*Equivalent circuit of a single-phase full bridge inverter.*

#### **Figure 10.**

*Equivalent circuit of a three-phase inverter.*

**Figure 11.**

*Equivalent circuit of a boost converter.*

common mode in use. Microgrids are designed to operate in both modes. Microgrids are designed such a way to get maximum power from renewable energy sources with wind, solar, and microturbines and able to fill its remaining required power from the grid. This mode of operation ensures maximum possible power to extract from local sources ensures the cost of energy cheapest. In the second mode, when the network gets isolated to faults, emergency conditions or natural calamities, the microgrid designed to operate in isolated mode. In the isolated mode, the entire power is supplied from locally available power sources of the microgrid.
